Just about numbers. A mutation is a change to a single individual that may or may not be propagated. For example a single point mutation where a base is replaced or deleted from a strand of DNA. This may cause the individual some discomfort or problems but is normally isolated to the individual.

A polymorphism is a distinct population of genes that have changed enough to create variation within a group. It is these changes that can lead to evolutionary changes since if this polymorphic change is beneficial it will continue to spread within the group as new offspring are produced.

I suppose a mutation may become a polymorphism if its success allows it to spread within a population!